Traditional CATV networks are designed to support one-way broadcast service using broadband coaxial cable in a tree-and-branch configuration. TV signals emanating from the headend at the root of the tree are broadcasted to the subscribers scattered at the extremities of the network. Taps and splitters are used to connect the branches while one-way amplifiers are spaced to compensate for signal attenuation. There are instances where two-way communications are considered, e.g., to gather customer viewing statistics or to support pay-per-view with remote addressability. Nevertheless, most CATV networks provide only one-way broadcast service.
The broadband potential of CATV coaxial cable plant coupled with the pervasive CATV networks is increasingly gaining recognition as one of the most cost effective architectural platform for providing interactive multi-media services. In addition to introducing new devices both at the subscriber end and at the central location, the CATV network has to be upgraded to allow two-way information flow in order to realize the broadband potential of the CATV network. The subscriber unit is for customers' access to multiple services. The central controller usually located at the headend coordinates the subscribers for two-way services. Both the subscriber unit and the central controller need to perform signalling, channel selection, modulating/demodulating among other functions. At the very least, the connectors used in the traditional CATV network, i.e., splitters and taps, have to be converted to the modem kind to allow signals traverse in both directions resulting in so called "two-way capable" network. Most proposals even require converting the one-way signal amplifiers to be two-way with the resulting network commonly called "two-way ready". Upgrading the traditional CATV networks not only requires significant investment but also demands careful engineering and ongoing maintenance effort.
Over the common transmission media, traffics traversed in different directions are carded in separate frequency bands. In the most common sub-split arrangement, the 5-30 MHz frequency band lower than the rest TV channel has been set aside for low rate data communication in the upstream direction (from the subscribers) while the downstream signals (to the subscribers) are carded in a number of TV channels in the 50-550 MHz range. This frequency allocation scheme on the broadband media not only occupies a number of CATV programming channels but also requires upgrading the traditional CATV networks. Other arrangements to split the frequency bands at a higher frequency partition (mid-split or high-split systems) require an additional device at the customer premise for frequency conversion in order to view TV programs on regular TV sets.
A serious shortcoming of most of the current approaches to two-way communication over CATV networks results from their designs to transport the two-way signals all the way back to the headend. Due to stringent bandwidth requirement of multi-media applications, the available bandwidth based on the sub-split frequency allocation scheme can only support a small number of subscribers. For example, using the similar modulation scheme as the cordless telephones, each channel requires 25 KHz to support the telephone service. Therefore, the 5-30 MHz band set aside for the upstream traffic can only accommodate up to 1000 telephone connections for the entire network. Other problems, such as noise and propagation delay, make this arrangement especially limited and costly.
For communication in the upstream direction, CATV network in essence is a multiple access media. While the current approaches use time division multiple access (TDMA) scheme to resolve the multiple access from subscribers, most of them function as time division multiplexing (TDM) without concentration instead of taking advantage of the intrinsic capability afforded by the multiple access scheme. Frequency division multiple access (FDMA) is a mature technology and yields a number of advantages over TDMA when applied in this environment.
The advances in fiber optics technology has stimulated its increasing deployment in the telecommunication industry. The current fiber optics technology in the subscriber loop section, called fiber-in-the-loop (FITL) or fiber-to-the-curb (FTTC), places an optical node serving typically fewer than ten subscribers. The drop section from the optical node to the customer premise still uses twisted pair cables. As a result, the cost is quite high for a handful of users to share the seemingly unlimited bandwidth of a optical fiber while a dedicated transmission means acting as the bottleneck of the whole distribution plant is still used to each subscriber.
On the other hand, the current trend in the CATV industry to use the optical fiber in a star topology for the CATV networks, called fiber star, improves the situation with noise and maintenance problems for serving up to tens of thousands of households at each fiber hub. To support the new two-way multi-media communication, there is a need to reduce the cost and to lay a sound foundation for growth. The CATV coaxial plant of the hybrid fiber/coax (HFC) is important for sharing the cost associated with the fiber optics equipment and for providing direct access of the broadband media to each of the subscriber, and in the mean time provides an extensible architecture for future growth.
The present invention overcomes the aforementioned limitations with the following objects:
Compatibility with two-way capable CATV network; PA0 Flexible and extensible system architecture; PA0 Low cost equipment and ease of engineering; and PA0 Incremental cost as system grows. PA0 No need to construct two-way ready CATV networks; PA0 Cost sharing and assess of the broadband media; PA0 Extensibility for support of new services and increased demand; PA0 Incremental cost for system growth; and PA0 Improved reliability for not relying on two-way amplifiers.
Further objects and advantages of the present invention will become apparent from a consideration of the drawings and ensuing description thereof.